The energy budget and its dynamic balance are key issues to optimize the performance of agile optical telecommunication networks. This requires the use of variable optical attenuators (VOAs). At the same time, an optical telecommunication line needs more optical amplifiers if the optical losses of the network are high. It is therefore very important to choose optical components (e.g., VOAs) with minimal losses when they are used in their transparent state. The in-guide (or guided wave) modulation of light propagation (light remains in the original waveguide while being transformed) appears to be the best solution for the fabrication of low-loss optical components. The main underlying principle here is the use of the evanescent part of light to affect it dynamically.
The material issue appears to be inherently involved in the development of evanescent field devices since we must provide media that is able to dynamically alter the optical properties of the mentioned above guiding components. The traditional way to do this is the change of the optical properties of the cladding of the guide, particularly in the area where the evanescent field propagates. In the case of the VOA application, the materials used must have relatively high sensitivity to the external excitation (e.g., thermal) to provide sufficiently high refractive index changes (compared to the waveguide's core nco and cladding ncl refractive indexdifference Δn) generating thus corresponding light attenuation.
Composite materials, including polymers, liquid crystals, organic/inorganic compounds, etc., are good candidates to address this paramount problem. These materials often provide low-cost fabrication techniques and are easy to manipulate in comparison with complex and costly operations needed to fabricate semiconductor components or micro electro mechanical systems.
Significant efforts have been devoted in the past for the development of robust and reliable composite materials for applications in evanescent field devices. However, an important problem persists with these materials, which is related to their dispersion (refractive index dependence upon light wavelength: n(λ)). Namely, in FIG. 1a (PRIOR ART), the wavelength dependence (see solid line) of the effective refractive index neff of a silica glass based fiber waveguide is schematically presented. The typical dispersion curve (see dashed line) of an original (“pure”) polymer material is also presented in the same FIG. 1a. In the case of modulation of the refractive index of the polymer np (for example, by changing the temperature T), the dashed line usually is shifted vertically while remaining parallel to its initial direction (this shift is not shown in FIG. 1a). When such a polymer is used as a part of waveguide's cladding, the light attenuation is achieved when we approach the situation np(T)≧neff(T), the maximum of attenuation being achieved for np(T)=nco(T). FIG. 1b is an experimental demonstration of such an attenuation transfer function.
However, it may be easily imagined that, because of the different slopes of the solid and dashed curves, the shorter (“blue”) wavelengths of the guided light will suffer from different losses compared to longer (“red”) wavelengths since the condition np(T)=neff(T) is satisfied at different values of T for the “red” and “blue” light. This will create an undesired tilt in the attenuation dependence upon the wavelength, or so called, wavelength dependent loss (WDL). For a more precise example, the maximum attenuation for the 1600 nm light will require a value of T for which the attenuation of 1500 nm will be much less, generating thus an undesired spectral tilt of attenuation, which may be appreciated from FIG. 1c. The curves 1-4 correspond to different levels of attenuation. The curve 4, for example, corresponds to 18 dB attenuation level at 1550 nm and it is accompanied by almost 6 dB WDL. This correspondingly will introduce additional changes in the spectral characteristics of the transfer function of the module where the VOA is integrated. A critical example is the Erbium Doped Fiber Amplifier. The problem is particularly important when multiple channels (for example, in the conventional C band) are propagating in the same device. Then the dynamic adding or dropping of a part of those channels brings to significant power fluctuations. Then the power equilibrium must be re-established in a spectrally uniform way to avoid the degradation of the signal to noise ratio for all remaining channels having various wavelengths.
Various methods have been proposed in the past to avoid such wavelength dependence. One of them is the change of the device's wavelength dependence by modifying the waveguide's geometry, e.g., via tapering the fiber or using multiple cladding structures, keeping however the material composition of the guide unchanged. Such a method is described in the U.S. Pat. No. 6,370,312B1, (Gregory A. Wagoner, Kevin J. McCallion, Gary O. Jameson, all with MOEC) granted on Apr. 9, 2002, “Fiber Optic Attenuation Systems, Methods of Fabrication Thereof and Methods of Attenuation Using the Same”. Another description of a similar solution may be found in the article, by Michel Monerie, “Propagation in Doubly Clad Single-Mode Fibers”, IEEE Journal of Quantum Electronics, Vol. QE-18, N4, April 1982. A device with good WDL characteristics is described also in the article entitled “Fused fiber optic variable attenuator” published in OFC 2000 Proceedings, 4. pp. 22-24 by V. Morozov, H. Fan, L. Eldada, L. Yang, Y. Shi.
However, these approaches require a special waveguide design and fabrication, which is rather complicated and costly. Note that in all these approaches, the light propagates a “direct” waveguide, that is, the waveguide's axis is not deformed. As we shall disclose below, the use of a non-uniform composite material as a part of cladding or the deformation of that axes may decrease the wavelength dependence of attenuation.
Another solution has been proposed, which uses a standard guide, such as a SMF28 fiber, along with a polymer cladding of modified chemical composition and refractive index n*p (see the dotted line in FIG. 1a). This composition contains absorbing (between the 1100-1200 nm range, see dashed line in FIG. 1d) organic dyes, which increase the dispersion tilt (solid line in FIG. 1d and dotted line in FIG. 1a) and provide a slope of n*p(λ) that matches the neff(λ) of the fundamental waveguide mode. In this case, the vertical shift of the curve representing the polymer's refractive index n*p(λ) brings to the simultaneous satisfaction of the condition n*p(T, λ)=neff(T, λ) for almost all channels (wavelengths) that are propagating in the fiber, and respectively, to the same attenuation due to the guided mode field diameter increase and further leakage. The U.S. Pat. No. 6,489,399 (K. P. Chan, D. G. Gascoyne, J. L. Krahn, G. A. Wagoner, all with MOEC) granted on Dec. 3, 2002, “Dye-appended polymers for broadband fiber optic devices”, and the references therein describe the details of such approach.
This method seams to be an easier way (compared to the fabrication of guides with special geometry), however, key tradeoffs must be considered here also. First one is the problem of the chemical miscibility and stability of the organic dye-polymer composition. This is the reason why only very small fractions of weight percentages of a specific group of organic dyes may be introduced in the polymer matrix. Another problem is related to the fact that the limited group of known dyes, which offer the “flattening” capacity, is composed of big organic molecules. Such molecules are rather unstable from photochemical point of view, in particularly when using multiple cycles of thermal control. The third problem is related to the additional optical losses introduced by those dyes at working telecommunication wavelengths (1300-1600 nm). Finally the high cost of the dye and the difficulty of its uniform integration in the polymer matrix (without aggregation or precipitation) complete the list of drawbacks of this approach.
In summary, the methods described above remain complex, non-reliable and costly. An ideal spectrally broadband device would use standard and cost effective optical waveguides (such as SMF28), be simply controllable and provide spectrally uniform (covering many communication channels simultaneously) attenuation of the optical signal with minimal loss and maximal material stability.
In view of the above, it is obvious that there is a need for a technology apt to spectrally design and control the attenuation of a light signal. More precisely, there is a need for a specifically designed controllable component (including material and geometry) for the broadband attenuation of the guided light.